Many diseases are from invading microorganisms thriving(Infections) in a weakened immune system?

"Vitamin deficiencies remain major etiological factors in the global burden of disease, especially in low- and middle-income countries."
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6775441/

"It has since been established that the complex, integrated immune system needs multiple specific micronutrients, including vitamins A, D, C, E, B6, and B12, folate, zinc, iron, copper, and selenium, which play vital, often synergistic roles at every stage of the immune response. Adequate amounts are essential to ensure the proper function of physical barriers and immune cells; however, daily micronutrient intakes necessary to support immune function may be higher than current recommended dietary allowances."
https://pubmed.ncbi.nlm.nih.gov/31963293/

"An enormous number of investigations point to the vital importance of various micronutrients in the interactions between the host immune system and viruses, including COVID-19. There are complex and multifaceted links among micronutrient status, the host immune response, and the virulence of pathogenic viruses"
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7647394/

"Micronutrients, often referred to as vitamins and minerals, are vital to healthy development, disease prevention, and wellbeing."
https://www.cdc.gov/nutrition/micronutrient-malnutrition/micronutrients/index.html

"“Germ theory” suggests that germs are what we need to worry about and we need to keep finding ways to kill them off. Whereas “terrain theory” argues that if the body is well and balanced, then germs that are a natural part of life and the environment will be dealt with by the body without causing sickness."
https://www.nutritionist-resource.org.uk/memberarticles/germ-theory-vs-terrain-theory-in-relation-to-the-coronavirus